A blowout preventer (BOP) is used to seal off a wellhead during drilling operations when the drill pipe experiences a "kick" or sudden increase in downhole pressure which threatens a blowout. The BOP functions by closing the annulus or gap between the interior drill pipe and the exterior surface pipe. Once the wellhead has been sealed, the downhole pressure must be overcome before drilling operations can resume.
A choke and kill line is used to circulate drilling mud to the wellhead. The drilling mud is "heavied" with clays which gives it a higher density so as to overcome the pressure. The higher density mud displaces the lower density mud in the line which is usually "gas cut" or aerated with gas from the downhole formation. In one arrangement, the drilling mud is pumped through the choke and kill line into the annulus between the surface casing and the well bore casing. The mud is displaced through the annulus and retrieved at the surface. This requires a connection between the annulus and the surface. Alternately, the higher density mud could be pumped through the annulus and retrieved through the choke and kill line. Further arrangements, use a separate choke line and a kill line. The mud is pumped through the kill line and returned through the choke line, bypassing the closed valves of the BOP.
Irrespective of the number of or types of choke and kill lines used, the fluids flowing through a choke and kill line are at relatively high pressures in order to overcome the downhole pressure. This high pressure imparts great separation forces to the connector between the choke and kill line and the wellhead. In order to keep the line in place, these forces are typically borne by a massive structure such as the one that supports the BOP stack.